Structure, properties and uses
Polymerspolymer: A polymer is a large molecule formed from many identical smaller molecules (monomers). have properties which depend on the chemicals they are made from, and the conditions in which they are made. For example, poly(ethene) can be low-density or high-density depending upon the catalystcatalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself. and reaction condition used to make it. The table summarises some differences in their properties:
|LDPE low-density poly(ethene)||HDPE high-density poly(ethene)|
|Branches on polymer molecules||Many||Few|
|Maximum useable temperature||85°C||120°C|
Thermosoftening polymers soften when heated and can be shaped when hot. The shape will harden when it is cooled, but can be reshaped when heated up again. Poly(ethene) is a thermosoftening polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material.
Thermosetting polymers have different properties to thermosoftening polymers. Once moulded, they do not soften when heated and they cannot be reshaped. Vulcanised rubber is a thermosetthermoset: Material that becomes permanently hard after heating. used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily.
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